180
Plates 2Q-32
The Thrush Nightingale Luscinia luscinia and the Nightingale L. megarhynchos together form a West Palearctic species group which provides one example of so-called 'sibling' species partnership (i.e. two species, each perfectly distinct in the biological sense and never hybridising*, but very difficult to separate in the field). It is believed that the distinction between the two was caused by geographical isolation during the last glacial period.
The Thrush Nightingale breeds in boreal, temperate and steppe climatic zones of east Europe and west Asia, within limits bounded approximately by the July isotherms of i6°G in the north and 25°C in the south. The most north-westerly breeding sites are in Finland, southern Sweden, Denmark and Baltic Germany, from where the westerly boundary runs south-east through Poland, east Czecho­ slovakia, Hungary, north-east Yugoslavia and Romania. The eastern distribution extends from this boundary to about the 90°E meridian in the central USSR (Voous i960). The species is a summer visitor to these regions, wintering in tropical east Africa, mainly south of the equator, as far as the Transvaal and Natal. It is a rare vagrant to Britain where, up to the end of 1975, there have been 28 records
(24 in spring, four in autumn). It is remarkable that before 1965 there were only three accepted reports of the species (15th May 1911,
10th May 1957 and 15th to 17th May 1958—all on Fair Isle).
Since 1965, however, records have increased dramatically and dur­ ing 1970 alone seven birds were identified. Most occurrences have been on Fair Isle, but individuals have also appeared in two parts of Shetland (three), in the Western Isles, in Fife (three), Northum­ berland (three), Yorkshire (two) and Kent. The spring records range from 8th May to 5th June, and there is one interesting one of an immature trapped on Fair Isle on 15th June; the earliest autumn record is on 31st July, and the latest on 2nd October. Since almost all the British records have occurred in the last ten years, one wonders whether the increase is real or whether it merely reflects the greater numbers of observers and the improved efficiency of recording techniques—particularly by trapping. The western
♦Although there is no record of hybridisation in the wild, there is one of a fertile cross in captivity which produced five young (Orn. Mber., 5 1 : 1-4).
[Brit. Birds, 6 9 : 265-271, J u l y 1976]
265

266 Thrush Nightingale studies extension of the Thrush Nightingale's range in recent times (see page 269) may account to some extent for the increase in reports, but perhaps the second factor is also significant. The Rarities
Committee regard the spring occurrences as 'evidence of the over­ shooting of Scandinavian adults in late May' (Smith et al.
1975).
In appearance the Thrush Nightingale is very like the Nightin­ gale; it has the same shape, is the same size of i 6 | cm (6Jin.), its plumage is superficially the same and its behaviour very similar.
However, given a clear view and good light, the spotted breast— particularly the sides—provides the best identification feature
(plate 32b). In my experience, this spotting is always present to some degree, although in some individuals the breast might be better described as mottled, scaly or barred rather than spotted. Because it is often difficult to obtain more than a succession of fleeting glimpses of this bird, it might require quite a long period of watching to establish the identity, beyond doubt, by breast markings alone.
There are other identification features, however. The upperparts
(except uppertail coverts) lack the rufous tinge of the Nightingale's plumage, and have been variously described in the literature as earth-brown, olive-brown or grey-brown. Of the illustrations in the standard works, that in The Hamlyn Guide is, in my opinion, the most helpful in terms of colour. Usually the upperparts look darker than the Nightingale's, but this is not reliable because degrees of shade are so dependent upon ambient light conditions. However, the breast and belly (but not the throat) of the Thrush Nightingale invariably look darker than the corresponding underparts of the
Nightingale, and most individuals have whitish flecks on the crown, nape and mantle. Some authorities maintain that the tail of the
Thrush Nightingale is significantly less chestnut in colour than that of its sibling partner, but I would never regard this as a usable field identification feature. I have more than once been able to observe both birds within my field of view at one and the same time and have always found it impossible to detect any real difference in tail coloration. However, the greyer, non-rufous back and wings of the
Thrush Nightingale have usually been quite obvious, as has also
(an important point) the contrast in colour between the back/wings and the tail. It must be emphasised, though, that field identification by appearance alone can be very difficult.
When examined in the hand an infallible identification feature is the difference in wing-formula from that of the Nightingale (well illustrated in The Handbook but The Hamlyn Guide is in error). The
Thrush Nightingale has a short first primary (the Nightingale's first primary is longer than the wing coverts); the second primary is longer than the fourth (it is shorter in the Nightingale); only the third primary is emarginated (the Nightingale's fourdi is also

Thrush Nightingale studies 267 emarginated).
The nestlings and juveniles are virtually indistinguishable from those of the Nightingale (plate 29), although the greyer, less rufous upperparts of young Thrush Nightingales would probably become increasingly obvious with age. I have seen very little of the juveniles after their first moult, which occurs shortly after they leave the nest, at about 18 days old (Berger 1967). Adults moult just before depart­ ing on their migration in late August.
Although identification of the Thrush Nightingale by appearance is difficult, identification by voice is altogether a different matter.
There is a strong similarity between the songs of the two species but the differences are quite distinct and can be quickly learned by any observer possessing even a relatively insensitive 'ear'. There has been at least one interesting attempt to record the song in musical notation (Stadler 1958, 1959), but it has been more usual for authors to give phonetic interpretations or prose descriptions, although a few of the latter in the standard works of reference are somewhat contradictory, for example: 'the song is altogether more solemn and more beautiful than the Nightingale's' ( The Handbook); 'the song[s are] equally musical' (the Field Guide) ; 'song resembles that of the Night­ ingale . . . but [is] less musical' {The Hamlyn Guide) (my italics).
Some rather more objective and helpful characteristics of the song are that the Thrush Nightingale's utterances are even louder and more powerful than the Nightingale's, which is often enough to differentiate between them. It has considerable carrying power and when the bird sings its whole frame vibrates with energy and the chestnut tail shivers (the Nightingale, of course, performs in like manner). Secondly, the Nightingale's familiar, flute-like
'pew-pew-pew' crescendo is never part of the Thrush Nightingale's song; the latter species can be immediately ruled out on hearing this phrase. Thirdly, the powerful 'jug-jug-jug' occurs regularly in both songs, but the Thrush Nightingale's version is invariably much stronger and has a character of its own; again, it can be diagnostic.
The most important characteristic of all, but one which, as far as I know, has never been described (surprisingly, as it is a completely diagnostic feature of the Thrush Nightingale's song), is a distinctive, frequently recurring, trisyllabic phrase, which may be written phonetically as 'chiddy-ock' and is usually repeated two or three times in succession. I first identified this phrase some 19 years ago and have since pointed it out to several ornithologists who knew the
Thrush Nightingale well. All of them later confirmed how valuable it had proved because it is never included in the Nightingale's repertoire. I strongly recommend, therefore, that anyone wishing to learn the voice of the Thrush Nightingale listens to a recording of the song and familiarises himself with this phrase. I have made many

268 Thrush Nightingale studies magnetic tape recordings of the song and have listed one in the references (Orr 1969).
I have studied the Thrush Nightingale in regions of West
Germany where both siblings occur together, and frequently been able to listen to the two songs simultaneously and make comparisons.
The Thrush Nightingale, like the Nightingale, sings during the night as well as by day and often gives its best performances between 2.0 and 4.0 a.m. It seems less susceptible to inclement weather than its counterpart and will sometimes continue to sing after a drop in temperature has caused the Nightingale to give up. The song period is short, commencing (in north-west Germany) during the second week of May, and finishing before the end of June, but song has also been heard in the winter quarters and there are several accounts of this in the literature. Call notes include an alarm croak, uttered when disturbed at the nest with young (rarely with eggs); a rather high pitched 'wheet' or 'seep', somewhat reminiscent of the call of the Chaffinch Fringilla coelebs; and a loud 'tak'. All are very like the corresponding notes of the Nightingale.
Thrush Nightingales arrive at their northern breeding stations at the end of April or beginning of May—in Sweden the majority turn up between 6th and 15th of the latter month (Rendahl 1961)— and they leave again for Africa in August or September. They are single-brooded. Nesting commences shortly after the birds settle in their breeding territories and most have begun laying before the end of May. Incubation usually starts during the first few days of
June in north-west Germany and is for a period of 13 days, most eggs hatching during the second half of that month. The young leave the nest eleven days later, although they are not fully fledged until about three weeks after hatching (Weber 1955). The hen seems to do all the incubating and she sits very tightly. When she does leave, she usually slips off quietly and runs from the nest, before flying low. Both parents take part in feeding the young and at the later stages of fledging they often become very bold indeed. The commonest clutch is five eggs, which for all practical purposes are identical in colour, shape and size with the Nightingale's, olivegreen or olive-brown all over (plate 32a).
The typical nesting habitat of the Thrush Nightingale is a damp, deciduous copse of alder Alnus glutinosa or birch Betula, including gardens if they provide this type of cover. It is frequently believed that there is a clear-cut distinction in habitat preference between the Nightingale and Thrush Nightingale in regions where they occur together, the former supposedly choosing dry situations and the latter preferring moist sites. This concept, may be generally correct over the entire breeding range but it certainly cannot be regarded as an infallible guide to nesting sites in north-west Ger-

Thrush Nightingale studies 269 many, where the two species often breed in almost identical situa­ tions. The Thrush Nightingale's nest, however, is almost always within about 20 metres of water and, indeed, is sometimes as close as 5 metres or so to a lake edge. On the other hand, I knew a regular site which was about 300 metres from the nearest water, in a perfectly dry situation.
The nest is bulky, like the Nightingale's, and is almost always on the ground. Normally it has a thick pad of leaves as a base, the cup being constructed of dead grass, leaves and bents which sometimes include aquatic vegetation such as Phragmites and Iris pseudacorus; this is lined with finer grasses. It is perhaps worthy of note that all the nests I have found have been enclosed between dead branches or largish sticks lying on the ground (plates 30, 31). Also, all but one of the sites were screened from the sky by an overhead canopy of foliage, whereas the Nightingale's nest is quite frequently placed in the open. The nest of the Thrush Nightingale is sometimes fairly difficult to find, at least during the eggs stage, although not all are well concealed and, if anything, it is easier to locate than the
Nightingale's and certainly not in the same category of difficulty as that of the closely related White-spotted Bluethroat L. svecica cyanecula, for example, which often nests in ostensibly the same type of situations.
During this century, and particularly since about 1940, the
Thrush Nightingale has extended its breeding range north and west. Until the 18th century it had been found in the neighbourhood of Stockholm, but by the middle of the 19th had disappeared from that region and did not return until about 70 years later. Since then there has been a continued expansion of its range north and west, and its numbers appear to have increased steadily (Karvic 1952,
Bjarvall 1965, Elofson 1968). It has been reported singing in Nor­ way during the present decade (Mork 1968).
The accompanying map (fig. 1) shows the distribution of the
Thrush Nightingale and Nightingale in the West German pro­ vince of Schleswig-Holstein, the region in which I have studied the species. This is a particularly interesting area for such a study as it is the only part of western Europe where the ranges of the two overlap. The map also shows the western advance in breeding range of the Thrush Nightingale during 1964-74, and I am very grateful to Dr G. A. J. Schmidt, of Kiel, for providing the infor­ mation on distribution for the last of these years, although he emphasises that the limit shown is only an approximate one (at my suggestion he kindly initiated a survey of singing males during the 1974 breeding season and collated the resulting observations).
The map information for 1964 is after Beckmann (1964). In the vicinity of Hamburg the western advance appears to have been

270
fM^Bm Breeding range of Thrush Nightingale in 1964
+ + + Northern limit of range of Nightingale in 1964
000000 Estimated western limit of Thrush Nightingale in 1974
Approximate further extension of Thrush Nightingale in south
0 - 0 - 0 - i n
1974,
Fig. 1. Breeding distribution of Thrush Nightingale Luscinia luscinia and Nightingale
L. megarkynchos in Schleswig-Holstein

Thrush Nightingale studies 2 7 1 particularly marked (Leuschner 1974).
One study area of mine is a compact zone of approximately 11 ha, where there have been twelve regular breeding sites of the Thrush
Nightingale and/or the Nightingale since before the mid 1950's.
Although not all sites are occupied every year, at least ten usually are. When I first came to know the area, in 1957, there were rather fewer Thrush Nightingales than Nightingales breeding there, but, by the late ig6o's, the Thrush Nightingale outnumbered the
Nightingale by approximately three to one. In very recent years, however, there has been a reversal in this trend: in 1973 the numbers breeding were seven Thrush Nightingales and four Nightingales; and, in 1975, four Thrush Nightingales and six Nightingales. The significance of this change is difficult to appraise at present, but it may represent only a cyclic fluctuation.
R E F E R E N C E S
BECKMANN, K. O . 1964.
Die Vogelwelt Schleswig-Holsteins.
Neumiinster.
BERGER, W. 1967. 'Die Mauser des Sprossers'.
J. Orn, 108: 320-327.
BJARVALL, A. 1965. 'The Thrush Nightingale in the neighbourhood of Stockholm'.
Vdr FdgelvSrld, 2 3 : 294-300.
BRUUN, B. 1970.
The Hamlyn Guide to Birds of Britain and Europe.
Feltham.
ELOFSON, O . 1968. 'Bird observations from the province of Medelpad'.
Vdr
Fdgelvarld, 27: 346-348.
GROTE, H . 1936. 'Die Winterquartiere von Nachtigall und Sprosser'.
Orn.
Mber., 44: 97-100.
KARVIC, N. G. 1952. 'The Thrush Nightingale in Dalsland'.
Vdr Fdgelvdrld, 11: 80.
LEUSCHNER, C. 1974. 'Der Sprosser im Hamburger Raum'.
Hamburger Avifaun.
Beitrdge, 12: 27-36.
MORK, J . 1968. 'Nattergal pa Sunnmore'.
Sterna, 8: 194, 202.
NIETHAMMER, J . 1937.
Handbuch der Deutschen Vogelkunde.
Leipzig.
O R R , N . W. 1969. 'Song of Thrush Nightingale'. Tape recording, British Library of Wildlife Sounds, 29 Exhibition Road, London SW7.
PETERSON, R., MOUNTFORT, G., and HOLLOM, P. A. D . 1954.
A Field Guide to the
Birds of Britain and Europe.
London.
RENDAHL, H . 1961. 'Die Fruhingsankunft des Sprossers in Schweden'.
Vogelwarte,
2 0 : 282-287.
SMITH, F. R., et al.
1975. 'Report on rare birds in Great Britain in 1974'.
Brit.
Birds, 68: 324.
STADLER, H . 1958 and 1959. 'Das Lied von Nachtigall und Sprosser'.
LOTUS, 9-10:
196-207; 1 1 : 131-139.
Voous, K. H . i960.
Atlas of European Birds.
London.
WEBER, H. 1955. 'Zur Verbreitung von Nachtigall und Sprosser in Mecklenburg
'954' J- Om., 96: 207-209.
WITHERBY, H. F., et al.
1938.
The Handbook of British Birds.
London, vol 2.
N. W. Orr, 4. Denham Drive, Christckurch, Dorset

P L A T E 29. Adult Thrush Nightingale Luscinia luscinia standing on edge of nest as young beg for food, Denmark, J u l y 1965 (pages 265-271) (photo: lb Trap-Lind)

P L A T E 30. Adult T h r u s h Nightingale Luscinia luscinia at nest, Schleswig-Holstein
G e r m a n y , J u n e 1970 (photos: N. W. On), Above, with caterpillar for young twothree days old (not visible in p h o t o g r a p h ) ; below, note the whitish markings on the crown, n a p e a n d mantle, a character found in most adult individuals

P L A T E 31. Above, approaching the nest from within cover, the parent Thrush
Nightingale Luscinia luscinia raises and fans its tail as it bends forward towards its young. Below, still with a caterpillar in its bill, the adult pauses warily before releasing the food; both parents feed the young {photo: N. W. Orr)

P L A T E 32. Above, nest of T h r u s h Nightingale Luscinia luscinia with the normal clutch of five eggs, D e n m a r k , J u n e 1966 {photo: lb Trap-Lind). Below, perched on a branch, this T h r u s h Nightingale Luscinia luscinia shows the breast markings which help distinguish it from the Nightingale L. megarhynchos {photo: JV. W. Orr)